It has been known that basic dyes can be fixed onto cotton by making use of a tannin mordant. The usual mordant comprises tannic acid that has been insolubilized with tarter emetic (basic antimony oxide). The resulting antimony tannate is employed to fix a basic dye onto the cellulose (which is thought to be as an insoluble color lake). In the mordanting process the water utilized must be absolutely free of any iron or the shades are adversely affected, being duller as the result of the reaction of the tannin to form black complexes (See for example J.M. Matthew's, Application of Dyestuffs to Textiles, Paper, Leather and Other Materials, Wiley, 1920).
The prior process for mordanting of textile goods, specifically cotton cellulose is not presently being employed because the antimony salts are highly poisonous in nature and therefore have ecological concerns for the environment. The treated fabrics needed to be rigorously washed in order to remove any unfixed antimony otherwise blood poisoning could result when the fabrics were worn next to the skin. Again antimony salts in the wash effluent gives rise to environmental concerns.
Today's consumers are quite aware of the environment and the problems that are associated with the coloration of textiles with synthetic dyestuffs. There appears to be a genuine desire on the part of the consumer to obtain textiles that have their coloration obtained with natural and/or non polluting dyeing materials, methods and procedures.
At the present time, sulfur dyes are primarily utilized to obtain a black color on cellulosic goods. C.I. Sulfur Black 1 is one of the most widely used dyes in the world for this purpose. Sulfur dyes are intrinsically insoluble, but they dissolve in solutions of alkaline reducing agents from which they are substantive to cellulosic materials. Once on the fiber, they must be converted back to their insoluble pigmentary form by oxidation.
Some of the problems with sulfur dyed goods is a phenomena known as bronzing as well as acid tendering. In severe conditions of heat and humidity or with improper oxidation, some sulfur dyeings, notably black, can generate a small amount of sulfuric acid within cellulosic fibers leading to tendering.